Linear perfluoropolyethers having an improved thermooxidative stability

ABSTRACT

Linear perfluoropolyethers of formula:
 
T-O(CF 2 O) n (CF 2 CF 2 O) m (CF 2 CF 2 CF 2 O) r (CF 2 CF 2 CF 2 CF 2 O) s -T 1   (I)
 
wherein n, m, r, s are integers such that the polymer number average molecular weight is comprised between 700 and 100,000 and the n/(n+m+r+s) ratio ranges from 0.05 to 0.40, and respective preparation process by addition of a peroxidic perfluoropolyether of formula (III):
 
T 4 —O(CF 2 O) n′ (CF 2 CF 2 O) m′ (O) h -T 5   (III)
 
having a PO from 1.8 to 4, to a perfluoropolyether oil preheated at a temperature comprised between 150° C. and 250° C. and subsequent exhaustive fluorination of the obtained compound.

The present invention relates to perfluoropolyether oils (PFPE) having ahigh thermooxidative stability in the presence of metals, and thepreparation thereof.

Specifically the invention perfluoropolyethers have a highthermooxidative stability combined with a high viscosity index and witha low pour point value.

More specifically the invention perfluoropolyethers comprisesubstantially —C₂F₄O—, —CF₂O— units.

Perfluoropolyether oils obtaind by oxidative polymerization ofperfluoroolefins are known and marketed.

In particular perfluoropolyethers are known having formula:R_(f)O(C₂F₄O)_(p)(CF₂O)_(q)R_(f)′  (A)with R_(f) and R_(f)′ equal to or different from each other selectedfrom CF₃—, C₂F₅—, ClCF₂—, ClCF₂CF₂—; p and q are integers, p+q gives thenumber average molecular weight and p/q ratio ranges from 0.1 to 10. Theperfluoroethylenoxide —CF₂CF₂O— and perfluoromethylenoxide —CF₂O— unitsare statistically distributed along the polymeric backbone. Theseperfluoropolyethers are marketed by the Applicant as Fomblin® Z.

They are obtained by oxidative polymerization of tetrafluoroethylene,see for example U.S. Pat. No. 3,715,378 and U.S. Pat. No. 5,744,651,subsequent thermal treatment and exaustive fluorination of the endgroups, see for example U.S. Pat. No. 4,664,766.

The formula (A) perfluoropolyethers have a good combination ofproperties, in particular a high viscosity index (V.I.), higher than240, generally comprised between 250 and 360, a high thermal stabilityand a pour point lower than −50° C., generally comprised between −60 and−90° C. The combination of said properties makes the formula (A)perfluoropolyethers suitable as lubricants in a wide temperature range,in particular also at low temperatures.

However their thermooxidative stability in the presence of metals is notvery high. For this reason perfluoropolyether oils decompose when oneoperates at temperatures higher than 200° C. Therefore said oils have amore limited use, they generally require additives to operate at hightemperatures (see the comparative Examples).

The need was therefore felt to have available pefluoropolyether oilslike those of formula (A) (Fomblin® Z type) but having an improvedthermooxidative stability in the presence of metals, substantiallymaintaining the combination of the Theological properties of said oilsof formula (A) as the high viscosity index and the low pour point.

Perfluoropolyethers satisfying the above technical problem have beensurprisingly and unexpectedly found.

An object of the present invention are perfluoropolyethers having thefollowing formula:T-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₁  (I)wherein:

-   -   T and T₁, equal to or different from each other are selected        from CF₃—, CF₃CF₂—, C₃F₇—, C₄F₉—, ClCF₂—, ClCF₂CF₂—;    -   n, m, r, s are integers such that the number average molecular        weight is comprised between 700 and 100,000, preferably between        1,500 and 20,000;    -   the m/n ratio is comprised between 2 and 20, preferably between        2 and 10;    -   the (r+s)/(n+m+r+s) ratio is comprised between 0.05 and 0.2,        preferably between 0.07 and 0.2;    -   the n/(n+m+r+s) ratio ranges from 0.05 to 0.40, preferably from        0.1 to 0.3;        the perfluorooxyalkylene units being statistically distributed        along the polymeric backbone.

The invention compounds are utilized in the lubrication field, inparticular they have the same uses of the formula (A) commercialcompounds (Fomblin® Z) but they show a higher thermooxidative stabilityin the presence of metals. In particular the formula (I) compoundshaving a number average molecular weight in the range 1,500–20,000(viscosity between 10 cSt and 1,000 cSt at 20° C.), are used aslubricant oils showing a high thermooxidative stability.

The invention perfluoropolyether oils, with respect to the knownperfluoropolyether oils having a comparable viscosity, show an increaseof thermooxidative stability in the presence of metals of about 30° C.or higher.

The stability of the formula (I) compounds when used as lubricants canbe further increased by the addition of thermal stabilizers ofperfluoropolyethers. For example, perfluoropolyethers comprising thefollowing groups: phosphines, phosphates, phosphazenes, benzothiazoles,triazines, amines, substituted amines type, nitroderivative compounds,can be mentioned.

Furthermore the formula (I) invention compounds, besides beingcharacterized by a high thermooxidative stability, also show a highviscosity index (V.I.), generally higher than 250, preferably higherthan 290, combined with a pour point value lower than −50° C.,preferably lower than −60° C. This combination of properties allows theuse of the invention compounds as lubricants in a wide range oftemperatures, even at low temperatures.

Tests carried out by the Applicant have shown that compounds having thesame structure of formula (I) invention compounds but with values of atleast one of the above indicated ratios, outside the claimed ranges,characterizing the formula (I) compounds, do not show the propertycombination of the invention compounds, in particular thethermooxidative stability in the presence of metals.

A further object of the present invention are linear perfluoropolyethershaving the following formula:T₂-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₃  (II)wherein:

-   -   T₂, T₃, equal to or different from each other are selected from        —(CF₂)COF wherein z=0, 1, 2, 3, CF₃—, CF₃CF₂—, C₃F₇—, C₄F₉—,        ClCF₂—, ClCF₂CF₂—, wherein the total moles of the end groups        comprise from 0.5% by moles to 50% by moles of —COF groups;    -   n, m, r, s are integers such that the polymer number average        molecular weight is in the range 700–100,000, preferably        1,500–20,000;    -   the m/n ratio is comprised between 2 and 20, preferably between        2 and 10;    -   the (r+s)/(n+m+r+s) ratio is comprised between 0.05 and 0.2,        preferably between 0.07 and 0.2;    -   the n/(n+m+r+s) ratio ranges from 0.05 to 0.4, preferably from        0.1 to 0.3;        wherein the perfluorooxyalkylene units are statistically        distributed along the polymeric backbone.

By exhaustive fluorination of the formula (II) compounds it is possibleto obtain the formula (I) compounds.

The —COF containing compounds of formula (II) can be used to confer oil-and hydrorepellence to various materials, for example stone materials.

Besides, said compounds containing —COF end groups can be transformedinto other compounds with other functional end groups, for example —COOHby hydrolysis, or —COOR (with R═CH₃, C₂H₅, C₃H₇) by hydrolysis inhydroalcoholic medium. The latter compounds can be transformed intoother products having other functional end groups: for example aminic,alcoholic, aldehydic, salts, nitrilic, amidic type. See for example U.S.Pat. No. 3,810,874. These compounds are used in the surface treatment ofvarious substrata to confer thereto properties as for example hydro- andoil-repellence or reduction of the friction coefficient, or they can beused as comonomers for obtaining block polymers as for examplepolyesters, polyurethanes or polyamides for the use at low temperatures,etc.

The preparation process of the compounds of formula (II) comprising stepa) of the preparation process of the compounds of formula (I) isdescribed hereinafter.

Another object of the present invention is a process for the preparationof formula (I) PFPEs comprising the following steps:

-   a) preparation of the compound of formula (II) by addition, under    stirring, of the formula (III) peroxidic compound:    T₄—O(CF₂O)_(n)(CF₂CF₂O)_(m)(O)_(h)-T₅  (III)-    wherein T₄, T₅, equal to or different from each other, are selected    from CF₃—, CF₃CF₂—, —COF, —CF₂COF, XCF₂—, XCF₂CF₂— wherein X═Cl,    —OR″_(f) wherein R″_(f) is a C₁–C₃ perfluoroalkyl, having a    n′/(n′+m′) ratio from 0.05 to 0.25 and a h/(n′+m′) ratio from 0.1 to    0.3 and a PO (peroxidic content) content, defined as grams of active    oxygen/100 grams of compound, from 1.8 to 4, preferably from 2 to    3.8, to a reaction medium formed by a perfluoropolyether oil,    contained in a reactor, maintained at a temperature in the range    150° C.–250° C., preferably 230–250° C., so as to have a PO of the    reaction mixture between 0 and 0.5, preferably between 0 and 0.2, by    continuously extracting the reaction mixture and heating the    collected fractions not containing the starting perfluoropolyether    oil at temperatures comprised between 220 and 250° C. until complete    removal of the residual peroxidic groups, obtaining the compound of    formula (II);-   b) fluorination of compound (II) with the obtainment of compound of    formula (I).

Alternatively step a) can be carried out by using as reaction medium,instead of the perfluoropolyether oil, the previously obtained compoundof formula (II).

The peroxidic perfluoropolyethers of formula (III) are known compoundsand can be prepared by oxypolymerization of tetrafluoroethylene attemperatures comprised between −100° C. and −40° C., in the presence ofUV light and/or radical initiator. See for example U.S. Pat. No.5,744,651.

In step a) the compound (III) is added to the preheated reaction medium,preferably with a flow-rate comprised between 0.1 and 1.3 kg/h per Kg ofreaction medium.

The fluorination process of step b) is known and can be carried out withgaseous fluorine at temperatures comprised betwee 150 and 250° C. and/orphotochemically at temperatures comprised between −40° C. and +200° C.See for example U.S. Pat. No. 4,664,766.

After step b) it is possible to subject compound (I) to moleculardistillation to separate fractions having a different molecular weightuseful for the various uses in lubrication. In this way fractions can beprepared having improved thermooxidative features with respect to thecommercial fractions of Fomblin® Z.

As perfluoropolyether oils, to be used both in the preparation of thecompounds of formula (I) and (II), the compounds of formula (A) marketedas Fomblin® Z, or other perfluoropolyether oils as for example Fomblin®Y, Krytox®, Demnum®, can for example be used.

Some Examples follow for illustrative but not limitative purposes of thepresent invention.

EXAMPLES

Characterization

-   -   Kinematic viscosity        -   The kinematic viscosity measurements have been carried out            with Cannon-Fenske type viscometers previously calibrated at            20° C., 40° C. and at 100° C.    -   Viscosity index        -   The viscosity index determination is carried out by using            the kinematic viscosity data at 40° C. and at 100° C. by            applying the ASTM D 2270 method.    -   Thermogravimetric analysis (TGA)        -   The thermogravimetric analysis is carried out by using the            DU PONT 951 instrument, putting about 20 mg of the sample in            the platinum measurement cell and using a temperature            gradient equal to 10° C./min in air atmosphere.    -   Pour Point        -   It has been determined according to the ASTM D 97-66 method.    -   Number average molecular weight        -   It has been determined by ¹⁹F NMR analysis.    -   Peroxidic content (PO) determination by iodometric titration        -   The peroxidic content analysis of the perfluoropolyether oil            is carried out according to the following procedure: a            weighed amount of oil (some grams) is dissolved in about 20            ml of an halogenated solvent (CFC 113 or Galden® ZV 60), 1            ml of acetic acid and 30 ml of a sodium iodide solution at            5% in isopropyl alcohol are added. It is put under strong            stirring for about 15 minutes and the iodine developed from            the reaction with the peroxide is titrated with an aqueous            solution of sodium thiosulphate having a known titre, by            using a potentiometric titrator Mettler DL 40 equipped with            platinum electrode and reference electrode. The content of            peroxide PO is expressed in g of active oxygen (MW=16) for            100 g of oil.    -   Thermooxidative stability        -   As thermooxidative stability measure, both in the presence            and in absence of metals, it is meant the temperature at            which there is a loss of 50% by weight of the compound            determined by thermogravimetric analysis.

Example 1

195 g of a perfluoropolyether of formula (A) wherein R_(f), R_(f)′ are—CF₃, —C₂F₅, ClCF₂—, ClCF₂CF₂—, p/q equal to 0.75 and having numberaverage molecular weight of 12,000 are introduced in a 500 ml roundbottomed glass flask, equipped with stirring, dropping funnel, inlet fornitrogen and with a siphon for the gas and liquid outlet placed so thatthe level of the reactants in the flask is maintained constant.

It is put under stirring, and it is heated with an oil bath untilreaching a temperature of 230° C.

172 grams/h of a peroxidic perfluoropolyether of formula:T₄—O(CF₂O)_(n′)(CF₂CF₂O)_(m′)(O)_(h)-T₅  (IIIa)wherein the end groups T₄, T₅ are CF₃— (8%) and CF₃CF₂— (92%), thenumber average molecular weight is equal to 9,060, with h/(m′+n′)=0.16such that the peroxidic content (PO) determined by iodometric analysisis equal to 2.3 g active oxygen/100 g of perfluoropolyether and themolar ratio n′/(n′+m′)=0.229 with a content of ether units CF₂O— equalto 0.204 moles/100 g of perfluoropolyether, are fed into the reactor for10 hours, through the dropping funnel, contemporaneously fluxingnitrogen.

The collected compound in the first 6 hours is eliminated since itcontains perfluoropolyether of formula (A).

In the following 4 hours, 554 grams are collected of substantially nonperoxidic compound, which is subsequently heated in a stirred reactor,at 240° C. until the complete removal of the small amounts of residualPO.

537 g of compound having the following structure formula:T₂—O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₃  (IIa)are obtained, wherein the end groups T₂, T₃, equal to or different fromeach other, are CF₃— (7% by moles), CF₃CF₂— (86%), CF₃CF₂CF₂— (3%),CF₃CF₂CF₂CF₂— (3%), —CF₂COF (1%); the number average molecular weight isequal to 6,950 and the molar ratios of the chain units are respectively:m/n=2.23; n/(n+m+−r+s)=0.285; (r+s)/(m+n+r+s)=0.078.

The compound is introduced in a photochemical reactor, equipped with a150 W high pressure mercury lamp and subjected to fluorination at thetemperature of 50° C. with a fluorine flow equal to 5 litres/h. After 11hours the compound is discharged and by the analysis it results to bethe compound of formula:T-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₁  (Ia)wherein T and T₁, equal to or different from each other, are CF₃— (10%by moles), CF₃CF₂— (85%), CF₃CF₂CF₂— (2.5%) and CF₃CF₂ CF₂CF₂— (2.5%)having a number average molecular weight equal to 6,930 and with molarratios of the chain units respectively equal to m/n=2.23;n/(n+m+r+s)=0.285; (r+s)/(m+n+r+s)=0.078.

The so obtained compound is subjected to molecular distillation undervacuum at 280° C. obtaining two fractions: a distillate of 182 g equalto 34% and a residue of 354 g equal to 66%. The residue of formula (Ia)has a number average molecular weight of 11,600 and results to have aviscosity of 229 cSt at 20° C., a pour point of −60° C. and a viscosityindex (V.I.) equal to 295.

The characteristics of said residue as viscosity at 20° C. are of thesame order of magnitude of Fomblin Z 25.

Example 2

In a 500 ml round bottomed glass flask, equipped with stirring, droppingfunnel, inlet for nitrogen and a siphon for the gas and liquid outletplaced so that the level of the reactants in the flask is maintainedconstant, 160 g of a perfluoropolyether of formula:T₂-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₃  (IIb)are introduced, wherein T₂, T₃, equal to or different from each other,are CF₃— (8% by moles), CF₃CF₂— (87%), CF₃CF₂CF₂— (2%), CF₃CF₂CF₂CF₂—(2.5%), —CF₂COF (0.5%), having a number average molecular weight 7,000,wherein the molar ratios of the chain units are respectively: m/n=2.51;n/(n+m+r+s)=0.261; (r+s)/(m+n+r+s)=0.075.

It is subjected to stirring and heated with oil bath until reaching atemperature of 230° C.

Then 198 grams/h of a peroxidic perfluoropolyether of formula:T₄-O(CF₂O)_(n′)(CF₂CF₂O)_(m′)(O)_(h)-T₅  (IIIb)wherein the end groups T₄, T₅ are CF₃— (6% by moles) and CF₃CF₂— (94%),the number average molecular weight is equal to 9,700, withh/(n′+m′)=0.185 such that the peroxidic content (PO) is equal to 2.7 gactive oxygen/100 g of perfluoropolyether and the molar ration′/(n′+m′)=0.198 with a content of ether units —CF₂O— equal to 0.172moles/100 g of perfluoropolyether, are fed into the reactor for 6 hours,through the dropping funnel under nitrogen flow.

In six hours 840 grams are collected of substantially non peroxidiccompound which is subsequently heated in a stirred reactor, at 240° C.until complete removal of the small amounts of residual PO.

814 g of compound having the following structure formula:T₂-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)-T₃  (IIc)are obtained, wherein the end groups T₂, T₃, equal to or different fromeach other are CF₃— (8%), CF₃CF₂— (87%), CF₃CF₂CF₂-(2%) andCF₃CF₂CF₂CF₂— (2.5%), —CF₂COF (0.5%); the number average molecularweight is equal to 7,000 and wherein the molar ratios of the chain unitsare respectively: m/n=2.51; n/(n+m+r+s)=0.261; (r+s)/(m+n+r+s)=0.075.

Example 3

With the same modalities of the Example 1, 250 g of formula (A)perfluoropolyether of the Example 1 are charged in the reactor, it isheated at 230° C. Successively 115 g/h of the peroxidicperfluoropolyether of the Example 2 are fed for 14 hours.

The compound collected in the first 10 hours is eliminated since itcontains the formula (A) perfluoropolyether.

In the following 4 hours, 345 grams of non peroxidic compound arecollected, having the following formula:T₂-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₃  (IId)wherein the end groups T₂, T₃, equal to or different from each other,are CF₃— (6%), CF₃CF₂— (84%), CF₃CF₂CF₂— (4%), CF₃CF₂CF₂CF₂— (5%),—CF₂COF (1%); the number average molecular weight is equal to 6,350, themolar ratios between the chain units are respectively m/n=2.44;n/(n+m+r+s)=0.266; (r+s)−/(m+n+r+s)=0.086.

Example 4

The compound obtained in the Example 1 having molecular weight equal to11,600 after addition of 2% by weight of alumina (Al₂O₃) is subjected tothermogravimetric analysis (TGA). The alumina is usually employed asreactant to simulate the perfluoropolyether oil stability in thepresence of metals as a function of temperature.

The thermogravimetric analysis gives a value of T_(1/2), i.e. thetemperature at which there is a loss of 50% by weight of the compound,equal to 295° C.

Example 5 (Comparative)

The TGA analysis is carried out on a commercial Fomblin® Z 25 having aviscosity of 245 cSt at 20° C. and formula:R_(f)O(C₂F₄O)_(p)(CF₂ ⁰)_(q)R_(f)′  (A1)wherein the end groups R_(f), R_(f)′, equal to or different from eachother, are CF₃— (49% by moles), CF₃CF₂— (6%), ClCF₂-(28%), ClCF₂CF₂—(17%); the p/q ratio is equal to 0.64. The content of ether units —CF₂O—is equal to 0.672 moles/100 g of polymer.

The oil is additioned with 2% of alumina and subjected to TGA analysis,as in the Example 4, obtaning a value of T_(1/2)=260° C.

By comparing the results of the TGAs it follows that theperfluoropolyether oil of the Example 1 characterized in the Example 4results to have an increase of thermooxidative stability of 35° C. incomparison with formula (A) commercial perfluoropolyether oils.

Example 6

The compound obtained in the Example 1 having molecular weight equal to11,600, after addition of 2% by weight of alumina and 1% by weight ofstabilizer bis-dipropylamine perfluoropolyether having the followingstructure:(CH₃CH₂CH₂)₂N—CH₂CF₂O(CF₂CF₂O)_(a)(CF₂O)_(b)CF₂CH₂N(CH₂CH₂CH₃)₂and a number average molecular weight of 2,000 and wherein the a/bratio=0.67 is subjected to TGA analysis under the same conditions of theExample 4 obtaining a T_(1/2) value equal to 324° C.

Example 7 (Comparative)

The commercial Fomblin® Z 25 used in the Example 5 (comparative) ischarged with 2% of alumina and with 1% of the stabilizer used in theExample 6.

The mixture is subjected to TGA under the same conditions of the Example4 obtaining a T₁₁₂ value equal to 297° C.

The perfluoropolyether oil of formula (I) of the present invention,additioned with a stabilizer, results to have an increase ofthermooxidative stability of 27° C. in comparison with the commercialperfluoropolyether oils of formula (A) additioned with the samestabilizer.

Example 8

5.07 g of compound obtained in the Example 1 having molecular weightequal to 11,600 are charged with 107 mg of alumina (2% by weight on thepolymer) and heated at 200° C. for 24 hours.

At the end of the thermal treatment the perfluoropolyether results tohave an average molecular weight equal to 10,300 (reduction of 11% withrespect to the initial molecular weight) and a weight loss equal to4.9%.

Repeating the test but using Fomblin® Z 25 of the Example 5(comparative) it is noticed the complete degradation of the commercialperfluoropolyether (loss in weight=98%).

Example 9

The compound obtained in the Example 1 having molecular weight equal to11,600 is subjected to thermogravimetric analysis (TGA) in absence ofmetals.

The thermogravimetric analysis carried out in the presence of N₂ and airgives a T_(1/2) value respectively of 434° C. and 433° C.

Example 10

The distillate obtained by molecular distillation in the Example 1 (182g) was analyzed by NMR, thus showing the following formula:T-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₁  (Ib)wherein T and T₁, equal to or different from each other, are CF₃— (7.5%by moles), CF₃CF₂— (86%), CF₃CF₂CF₂— (3%) and CF₃CF₂—CF₂CF₂— (3.5%), thenumber average molecular weight is equal to 3,960 and the molar ratiosof the chain units are respectively equal to m/n=2.34;n/(n+m+r+s)=0.276; (r+s)/(m+n+r+s)=0.079.

The viscosity at 20° C. of said distillate is 31.5 cSt.

The compound after addition of 2% by weight of alumina (Al₂O₃) issubjected to thermogravimetric analysis (TGA) giving a T_(1/2) value of300° C.

Example 11 (Comparative)

With the same modalities of the Example 3, 250 g of formula (A)perfluoropolyether of the Example 1 are charged into the reactor whichis heated at 230° C. Successively 115 g/h of a peroxidicperfluoropolyether having the following formula:T₄-O(CF₂O)_(n′)(CF₂CF₂O)_(m′)(O)_(h)-T₅wherein the end groups T₄, T₅ are CF₃— (10%) and CF₃CF₂— (90%), thenumber average molecular weight is equal to 5,500, with h/(m′+n′)=0.15such that the peroxidic content (PO) is equal to 2.4 but having a molarratio n′/(n′+m′)=0.379 and a content of ether units —CF₂O— equal to0.381 moles/100 g of perfluoropolyether, are fed into the reactorthrough the dropping funnel for 14 hours, contemporaneously fluxingnitrogen.

The compound collected in the first 10 hours is eliminated since itcontains the perfluoropolyether of formula (A) previously charged in theflask.

In the following 4 hours, 320 grams of substantially non peroxidiccompound are collected and subsequently heated in a stirred reactor, at240° C. until the complete removal of the small amounts of residual PO.315 g of compound having the following structure formula:T₂-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₃are obtained, wherein the end groups T₂, T₃, equal to or different fromeach other, are CF₃— (9% by moles), CF₃CF₂— (86%), CF₃CF₂CF₂— (2%),CF₃CF₂CF₂CF₂— (2%), —CF₂COF (1%); the number average molecular weight isequal to 3,850 and the molar ratios of the chain units are respectively:m/n=0.85; n/(n+m+−r+s)=0.501; (r+s)/(m+n+r+s)=0.067.

The compound is then introduced in a photochemical reactor, equippedwith a 150 W high pressure mercury lamp and subjected to fluorination atthe temperature of 50° C. with a fluorine flow equal to 5 litres/h.After 11 hours the compound is discharged and by the analysis it resultsto be the compound of formula:T-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₁wherein T and T₁, equal to or different from each other, are CF₃— (10%by moles), CF₃CF₂— (86%), CF₃CF₂CF₂— (2%) and CF₃CF₂CF₂—CF₂— (2%),having a number average molecular weight equal to 3,840, a viscosity at20° C. of 30 cSt, and molar ratios of the chain units respectively equalto m/n=0.85; n/(n+m+r+s)=0.504; (r+s)/(m+n+r+s)=0.068.

The so obtained compound contains the same chain units of compound offormula (Ib) of the Example 10 and has comparable molecular weight andviscosity but the molar ratios m/n and n/(n+m+r+s) are outside theranges of formula (I) compounds of the present invention.

Example 12 (Comparative)

The compound of the the Example 11 (comparative) having a number averagemolecular weight equal to 3,840 and a viscosity of 30 cSt at 20° C.,after addition of 2% by weight of alumina (Al₂O₃), is subjected tothermogravimetric analysis (TGA). A value of T_(1/2) equal to 268° C. isobtained.

The comparison of this T_(1/2) (268° C.) with the T_(1/2) of thecompound of example 10 (300° C.) shows that linear perfluoropolyetherscontaining —CF₂O—, —CF₂CF₂O—, —CF₂CF₂CF₂O—, —CF₂CF₂CF₂CF₂O— units buthaving at least one of the molar ratios m/n, n/(n+m+r+s),(r+s)/(m+n+r+s) outside the ranges of formula (I) compounds of thepresent invention, have a thermooxidative stability, in the presence ofmetals, lower than that of the linear perfluoropolyethers of the presentinvention having similar viscosity.

1. A process for lubricating in the presence of metals, comprising thestep of applying linear perfluoropolyethers as lubricants, where saidlinear perfluoropolyethers have the following structural formula:T-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₁  (I)wherein: T and T₁ are the same or different, and are each selected fromthe group consisting of CF₃₋, CF₃CF₂₋, C₃F₇₋, C₄F₉₋, CICF₂₋, orCICF₂CF₂₋; n, m, r, s are integers such that the number averagemolecular weight is comprised between 700 and 100,000: the m/n ratio iscomprised between 2 and 20; the (r+s)/(n+m+r+s) ratio is comprisedbetween 0.05 and 0.2; the n/(n+m+r+s) ratio ranges from 0.05 to 0.40;and wherein the perfluorooxyalkylene units are statistically distributedalong the polymeric chain.
 2. A process for conferring water- andoil-repellance to surfaces, comprising the step of applying linearperfluoropolyethers to said surfaces, where said linearperfluoropolyethers have the following structural formula:T-O(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)-T₁  (I)wherein: end groups T and T₁ are the same or different, and are eachselected from the group consisting of CF₃₋, CF₃CF₂₋, C₃F₇₋, C₄F₉₋,CICF₂₋, CICF₂CF₂₋ or -(CFz)z COF, wherein z=0, 1, 2 or 3; n, m, r, s areintegers such that the number average molecular weight is comprisedbetween 700 and 100,000; the m/n ratio is comprised between 2 and 20;the (r+s)/(n+m+r+s) ratio is comprised between 0.05 and 0.2; then/(n+m+r+s) ratio ranges from 0.05 to 0.40; wherein theperfluorooxyalkylene units are statistically distributed alona thepolymeric chain, and wherein the total moles of the end groups have from0.5% by moles to 50% by moles of -COF groups.